It looks like the most concentrated way to get sulforaphane is from broccoli sprouts. I'm about to start sprouting some.

There's an alternative source: the moringa tree (also a cruciform) has leaves high in an isothiocyanate which has the same active chain as sulforaphane (but a larger 'tail' meaning it won't work exactly the same)

There's been interesting work with both of these in inducing a pathway that leads to excreting toxins like benzine (in air pollution from car exhaust).